(CNN) -- Fortune's latest ranking of America's 500 largest corporations includes more women CEOs than ever before.
Women lead 18 of the 500 companies, including Hewlett-Packard (10th) run by Meg Whitman and IBM (19th) run by Ginni Rometty, both of whom started within the last year.
Other executives to have shattered the glass ceiling include Patricia Woertz of agricultural processors Archer Daniels Midland; Indra Nooyi who is CEO of Pepsico, Irene Rosenfeld who heads up Kraft Foods; Ursula Burns who is CEO at Xerox and Sherilyn McCoy at Avon.
Rometty is IBM's first female CEO and Xerox chief Ursula Burns is the first African-American woman to head a Fortune 500 company. The top five female CEOs on the list all held positions in strategic planning before being appointed.
There are an additional 21 female CEOs in the Fortune 501-1000, some managing steel, oil and energy companies.
Lists like the Fortune 500 typically generate debate about how to improve female representation in governance positions.
Catalyst, a nonprofit research organization advocating for more women in business, found that in Canada women hold 14.5% of board seats, a figure that rose by only half a percent between 2009 and 2011. In America, women hold 16.1% of board seats, an increase of less than half a percent from 2010.
About 10% of the top 500 U.S. companies, and almost 40% of Canada's 500 largest companies have no women on their boards, according to Catalyst.
In the UK, where only 9.8% of FTSE250 companies are lead by women, and 44% have no female board members at all, a report by former trade minister Lord Davies estimated that, at the current rate of change, it will take 70 years to achieve gender balance in the boardroom.
Baroness Mary Goudie is founder of the 30% Club, which encourages company chairs to commit to achieving at least 30% women on UK corporate boards. She notes that, while there is a record number of women CEOs in the United States, "on the other hand, you've got hardly any women on boards.
"The United States doesn't have the same regulatory situation as boards in the UK and parts of Europe. In the UK you can only do two terms on a board, so we're seeing a turnaround for women, whereas in the States, people seem to get on a board and stay for a lifetime, almost. The pipeline is blocked forever."
Despite this, Baroness Goudie feels female CEOs can influence the make-up of boards. "They can try to influence the chairman of their company by saying, 'Look at the shortlist: How many women are you bringing on?'"
They may also indirectly benefit others aspiring to senior positions. "A number of women CEOs have children. They are looking to make change and to encourage the pipeline. There'll be a few who won't be helpful but the vast majority of women CEOs are doing all they can to encourage other women to go after these jobs."
Companies with fewer females in senior positions might be shooting themselves in the foot.
Catalyst has also analyzed the performance of F500 companies with women board members. It found that, from 2004 to 2008, the Fortune 500 companies with the most female board directors outperformed those with the least -- by 16% on return on sales and by 26% on return on invested capital.
Companies which had three or more women on boards for at least four of those years outperformed those with the lowest rates of female representation by 84% on return on sales, 60% on return on invested capital and 46% on return on equity.